In producing heat exchangers, e.g., radiators, there have been cases where a joint between two members comprising stainless steel is brazed with a brazing nickel.
As prescribed in JIS Z3265-1986, this brazing nickel is an alloy comprising nickel as the main component and additives such as boron, silicon, chromium, iron, and phosphorus.
These additives serve to regulate the melting point of the brazing material, to improve the flowability of the molten brazing material and the ability thereof to wet base materials, and to improve the toughness of the hardened brazing material.
When such a nickel-based brazing material solidifies, an .alpha.-phase of nickel crystallizes out as highly tough primary crystals and this crystallization begins at the interface with the base material. Hence, brittle phases comprising intermetallic compounds of other metals, e.g., silicon, boron, and phosphorus, are apt to crystallize out at the center of the bonding part and in inner parts and surfaces of the fillets.
If such brittle phases generate continuously in brazing parts, cracking is apt to occur in the brazing parts, in particular in the fillet parts (see FIG. 1). Since continuous brittle phases frequently concentrate in the finally solidified areas, cracks also concentrate therein. In particular, the finally solidified areas tend to develop long cracks extending to the surfaces of the fillets.
The brazing fillet parts which have developed long cracks extending to the fillet surfaces in products in which a liquid is in contact with the brazing fillet parts, e.g., in radiators, suffer penetration of the liquid into the long cracks extending to the fillet surfaces, resulting in crevice corrosion. It is hence desired to prevent the generation of such cracks extending to fillet surfaces.
A prior art technique which has been regarded as effective in eliminating brittle phases for coping with long cracks in brazing fillet parts is to reduce the fillet size and employ a higher brazing temperature and a longer brazing time.
However, it is difficult to prevent the crack generation with the above technique.
Furthermore, especially in the case of EGR coolers, there is a problem that since corrosive fluids such as high-temperature exhaust gas, cooling water, and exhaust gas condensate water come into contact with brazing fillet parts, these corrosive fluids penetrate into cracks in the brazing parts to cause crevice corrosion. In addition, the brittle phases themselves exposed on the fillet surfaces are susceptible to corrosion by corrosive fluids, and the corrosion of the brittle phases accelerates the crevice corrosion.